For the colour-changing treatment of keratin-containing fibres, for example human hair, wool or furs, use is usually made of two colouring methods. In the first method, the coloration is produced with so-called oxidative or permanent colorants, using a mixture of various developer substances and coupler substances and an oxidizing agent. If required, so-called direct (nonoxidative) dyes can be added to round off the colouring result or to produce particular colour effects. The second method uses exclusively direct dyes, which are applied to the fibres in a suitable carrier mass. This method is easy to use, exceptionally gentle and is characterized by low damage to the keratin fibres. The direct dyes used here are subject to a large number of requirements. For example, they have to be acceptable from a toxicological and dermatological point of view and allow the attainment of colorations in the desired intensity, which, inter alia, also requires adequate solubility in water. Furthermore, good lightfastness, acid fastness and rubbing fastness is required for the colorations achieved.
For a direct (nonoxidative) colorant for keratin fibres, a combination of different nonoxidative dyes is usually required. Since the choice of red and blue nonoxidative dyes which can be used in colorants for keratin fibres, in particular human hair, is limited, there is an urgent need to expand the dye palette which can be used.